Process of stabilization of anhydrous dibasic calcium phosphate against fluorine ions with cyclic aminophosphonic acids

ABSTRACT

A process for stabilizing anhydrous dibasic calcium phosphate against reaction with fluorine ions comprising treating an aqueous suspension of anhydrous dibasic calcium phosphate at a pH of from 5 to 10 with a cyclic aminophosphonic acid of the formula ##STR1## WHEREIN N IS AN INTEGER FROM 1 TO 3, OR A WATER-SOLUBLE SALT THEREOF, IN AN AMOUNT OF FROM 0.01% TO 5% BY WEIGHT WITH REFERENCE TO THE ANHYDROUS DIBASIC CALCIUM PHOSPHATE; AS WELL AS TOOTH CLEANING PREPARATIONS CONTAINING THE STABILIZED ANHYDROUS DIBASIC CALCIUM PHOSPHATE.

RELATED ART

Anhydrous dibasic calcium phosphate having the formula CaHPO₄ is apolishing substance frequently utilized in tooth cleaning preparationsas, for example, toothpastes and powders. For this purpose it may beused alone or in admixture with other polishing substances as, forexample, silica gel or plastics cleaning substances. If solublefluorides or other compounds providing fluorine ions are added to thesetooth cleaning preparations as anti-caries substances, the fluorinecontained therein may be inactivated by conversion into the soluble andinactive calcium fluoride. The process thereby occuring may berepresented by the following empirical reaction:

    CaHPO.sub.4 + 2 NaF → CaF.sub.2 + Na.sub.2 HPO.sub.4

the speed of this inactivation reaction is influenced by severalcircumstances such as temperature, pH value of the mixture and itscomposition.

For the use of a polishing agent as cleaning material in toothcleaningpreparations, its abrasive behavior is of decisive importance, sinceproducts to be used for this purpose must only have an abrasive powerwhich does not cause damage to the teeth. Owing to its favorableabrasive behavior, anhydrous dibasic calcium phosphate already enjoysgreat popularity as a cleaning material in tooth cleaning preparations.Its property of making the fluorine-containing compounds used forcontrol of caries in the tooth cleaning preparation inactive or at leaststrongly reducing their action is, however, extremely undesirable. Theproblem, therefore, exists of finding ways and means largely to stopthis inactivation of the fluorine-containing added substances.

OBJECTS OF THE INVENTION

An object of the present invention is the development of a process forstabilizing anhydrous dibasic calcium phosphate against reaction withfluorine ions consisting essentially of suspending anhydrous dibasiccalcium phosphate in an aqueous medium at a pH of from 5 to 10,containing from 0.01% to 5% by weight based on the content of dibasiccalcium phosphate of a cyclic aminophosphonic compound selected from thegroup consisting of (A) compounds of the formula ##STR2## wherein n isan integer from 1 to 3, and (B) water-soluble salts thereof, andseparating said stabilized anhydrous dibasic calcium phosphate.

Another object of the present invention is the obtaining of a stabilizedanhydrous dibasic calcium phosphate.

A further object of the present invention is the obtaining of toothcleaning preparations containing a stabilized anhydrous dibasic calciumphosphate.

These and other objects of the present invention will become moreapparent as the description thereof proceeds.

DESCRIPTION OF THE INVENTION

It has now been found that a satisfactory and easily effectedstabilization of anhydrous dibasic calcium phosphate against reactionswith fluorine ions is possible by treating the anhydrous dibasic calciumphosphate in aqueous medium at a pH of from 5 to 10, preferably from 6to 8, with a cyclic aminophosphonic acid of the general formula ##STR3##in which n is an integer of from 1 to 3, or a water-soluble saltthereof, in an amount of from 0.01% to 5% by weight, preferably from0.1% to 2% by weight, referred to the amount of anhydrous dibasiccalcium phosphate employed.

More particularly, the invention relates to a process for stabilizinganhydrous dibasic calcium phosphate against reaction with fluorine ionsconsisting essentially of suspending anhydrous dibasic calcium phosphatein an aqueous medium at a pH of from 5 to 10, containing from 0.01% to5% by weight based on the content of dibasic calcium phosphate of acyclic aminophosphonic compound selected from the group consisting of(A) compounds of the formula ##STR4## wherein n is an integer from 1 to3, and (B) water-soluble salts thereof, and separating said stabilizedanhydrous dibasic calcium phosphate; as well as the stabilized dibasiccalcium phosphate so produced and tooth cleaning preparations containingthe same.

In carrying out the process of the invention the cyclic aminophosphonicacid or its water-soluble salt, the anhydrous dibasic calcium phosphateand water may be admixed in any manner. For example, the cyclicaminophosphonic acid or its water-soluble salt may be used in themixture either as an aqueous solution or in solid form and the anhydrousdibasic calcium phosphate may be used in the mixture either as anaqueous suspension or in solid form.

The preparation of the aminophosphonic acids to be used according to thepresent invention, or their water-soluble salts may be carried out in asimple way by reacting dicarboxylic acid derivatives of the formula

    X -- (CH.sub.2).sub.n -- X

in which n represents an integer of from 1 to 3 and X represents anitrile group or --CONH₂, with phosphorus trihalides or mixtures ofphosphorus trihalides and phosphorous acid, hydrolyzing the reactionproduct and if desired converting into the salts, as are described incopending commonly-assigned U.S. patent application Ser. No. 498,996,filed Aug. 20, 1974 now U.S. Pat. No. 3,925,456.

The reaction may be carried out, for example, by first melting thedicarboxylic acid diamide with phosphorous acid and slowly adding PCl₃with stirring. The mostly viscous reaction product formed is thenhydrolyzed by addition of water. An addition of an acid is notnecessary, since the reaction product itself has an acid reaction.Starting from dicarboxylic acid dinitrile, however, this may also bedissolved in an inert solvent as, for example, dioxane or chlorinatedhydrocarbons, and then treated with phosphorus trihalide. After that,phosphorous acid is then added and after addition of water, the solutionis hydrolyzed. In the last-named process the phosphorous acid may alsobe omitted, if desired. Suitable phosphorus trihalides are especiallyphosphorus trichloride and phosphorus tribromide. The latter has beenfound particularly suitable when nitriles are used as the reactionpartner. The molar proportions of dicarboxylic acid derivative andphosphorus compound are 1:2 to 1:6, preferably 1:4. The above-mentioneddicarboxylic acid derivative used are those of malonic acid (n=1),succinic acid (n=2) and glutaric acid (n=3).

Advantageously the cyclic aminophosphonic acids may also be employed inthe form of their water-soluble salts such as their alkali metal salts,especially lithium, sodium and potassium salts, and their ammoniumsalts. The conversion into the salts may easily be carried out bypartial or complete neutralization with the corresponding bases.

The stabilization according to the present invention may either becarried out before isolation of the anhydrous dibasic calcium phosphatefrom the reaction medium in which it is prepared or in a later separatetreatment process. The preparation of the anhydrous dibasic calciumphosphate may be effected according to processes known from theliterature, for example, from calcium hydroxide and phosphoric acid. Inthis case it is advantageous for the preparation of the anhydrousdibasic calcium phosphate to be so controlled that rounded off crystalaggregates as homogeneous as possible are obtained.

If the stabilization is to be carried out on previously isolatedanhydrous dibasic calcium phosphate which is the preferred method ofproduction, this previously isolated anhydrous dibasic calcium phosphateis treated with an aqueous solution of the stabilizer, the pH of thesolution being adjusted to from 5 to 10, preferably from 6 to 8.However, even if the stabilization is effected before isolation of theanhydrous dibasic calcium phosphate from the reaction medium, theaqueous suspension is set at a pH of 5 to 10, preferably 6 to 8, withthe addition of the stabilizer. The amount of stabilizer required caneasily be found by testing. It has been found that in general 0.01% to5% by weight, preferably 0.1% to 2% by weight, based on the amount ofanhydrous dibasic calcium phosphate to be stabilized is sufficient. Theamount, within the indicated limits, is dependent on (a) the extent ofthe desired stabilization, (b) the particle size, surface and surfacestructure of the anhydrous dibasic calcium phosphate prepared, and (c)the time of contact between the stabilizer and the product to bestabilized. It has further been found suitable to use the water-solublesalts of the cyclic aminophosphonic acids, as for example, alkali metalsalts, especially sodium salts. If the free acids are to be used, it maybe necessary to correct for pH deviations, for example, by addition ofcalcium hydroxide or calcium oxide. Owing to the small amounts of theadded cyclic aminophosphonic acid, however, this is often unnecessary.The stabilizers to be used according to the present invention may alsobe used in combination with other substances, such as other stabilizers,aids to precipitation or protective colloids as, for example, withpyrophosphates, tripolyphosphates and other polymeric phosphates,polysilicates, polycarboxylates, lignin derivatives, gums andpolysaccharides.

Cyclic aminophosphates acids of the above-mentioned general formulawhich are also substituted on the nitrogen atom also show a stabilizingaction, but this is substantially less and, therefore, of littletechnical interest.

The present invention relates primarily to the preparation of ananhydrous dibasic calcium phosphate stabilized against reaction withfluorine ions, for use in tooth cleaning preparations. Such stabilizedproducts, however, may also be advantageous in other fields ofapplication. The tooth cleaning preparations to be prepared according tothe present invention may contain, in addition to the stabilizedanhydrous dibasic calcium phosphate serving as polishing material, theusual constituents such as, for example, thickeners, surface-activecompounds of tensides, emulsifiers, bactericides, and flavoringsubstances. A toothpaste is the preferred form of the tooth cleaningpreparations with a content of stabilized anhydrous dibasic calciumphosphate according to the present invention.

Toothpastes are generally pasty preparations based on water, whichcontain thickeners, wetting and foaming agents, moisture-retentionagents, polishing, scouring or cleaning substances, aroma-impartingsubstances, taste correctors, antiseptic and other substances valuableas mouth cosmetics. The content of polishing substances in thetoothpastes, i.e., the content of the anhydrous dibasic calciumphosphate which is to be used according to the present invention andwhich is stabilized against reaction with fluorine ions, will generallyvary from 25% to 60% by weight, referred to the total mass of thetoothpaste. The wetting and foaming agents employed are especiallysoap-free anionic surface-active compounds such as fatty alcoholsulfates, for example, sodium lauryl sulfate, monoglyceride sulfates,sodium lauryl sulfoacetate, sarcosides, taurides and other anionicsurface-active compounds which do not affect the taste, in amounts from0.5% to 5% by weight. For the preparation of the binder for toothpaste,all thickeners usual for this purpose may be used, such ashydroxyethylcellulose, sodium carboxymethylcellulose, tragacanth,carrageen moss, agar-agar and gum arabic, as well as additionally finelydivided silicic acids, all in amounts of from 0.1% to 5% by weight ofthe whole toothpaste. As moisture-retention means, glycerine andsorbitol are of principal importance, in amounts which may be up toone-third or from 5% to 331/3% by weight of the whole toothpaste. Wateris also present in amounts of from 10% to 50% by weight of the wholetoothpaste. With toothpowders, the water, thickeners andmoisture-retention means are omitted. The desired aroma and tasterequirements can be attained by an addition of essential oils such aspeppermint, clove, wintergreen and sassafras oils, as well as bysweetening agents, such as saccharin, dulcin, dextrose or laevulose.

As essential further component of the tooth cleaning preparation to beprepared according to the present invention comprises thefluorine-containing compounds serving for the control of caries or forcaries prophylaxis. These are present in amounts of from 0.01% to 2%measured as fluorine ions of the whole tooth cleaning preparations.

Fluorines against which the anhydrous dibasic calcium phosphate isstabilized are, for example, sodium fluoride, potassium fluoride,aluminum fluoride, ammonium fluoride, monoethanolamine-hydrofluoride,hexadecylamine-hydrofluoride, oleylamine-hydrofluoride,N,N',N'-tri-(polyoxyethylene)-N-hexadecyl-propylenediamine-dihydrofluoride,bis-(hydroxyethyl)-amino-propyl-N-hydroxyethyl-octadecylamine-dihydrofluoride,magnesium aspartate-hydrofluoride, and tin fluoride. Also fluorinecompounds in which the fluorine is present primarily in a preponderantlynon-ionic bond, which, however, may split off fluoride, for example, byhydrolysis or other chemical reactions, such as sodiummonofluorophosphate, potassium monofluorophosphate, magnesiummonofluorophosphate, indium fluorozirconate, zirconiumhexafluorogermanate, in combination with the anhydrous dibasic calciumphosphate treated according to the present invention are protected fromundesired loss of fluorine.

In the case of further cleaning and filling substances possibly alsoused in the tooth cleaning preparations, naturally those are preferredwhich themselves also do not inactivate fluoride, as for example,plastics particles, silica gels or pyrogenic silicic acids, or inorganicsubstances covered with polymers, waxes or other means or madecompatible towards fluoride in any other way.

With reference to the other constituents it is naturally advisable toalso choose those which do not inactivate the fluoride, in order thatthe advantage attained with the cleaning bodies according to the presentinvention is not reduced or eliminated, otherwise no special limits areset to the formulation of the tooth cleaning preparations.

Since the preparation of the product stabilized against the reactionwith fluorine ions is generally effected by treatment of a previouslyisolated anhydrous dibasic calcium phosphate with an aqueous solution ofthe stabilizer, it is, of course, also possible to stabilize ananhydrous dibasic calcium phosphate present in an already finishedtoothpaste subsequently against the reaction with fluorine ions by anaddition thereto of salts of the cyclic aminophosphonic acids. Suchmeasures may be employed in special circumstances, but should remainconfined to exceptions, since the result of such a difficultlycontrollable treatment in such a heterogeneous system as that of atoothpaste, is not always fully ensured.

The following examples illustrate further the present invention without,however, being restricted thereto.

EXAMPLES

First the preparation of some cyclic aminophosphonic acids to be used asstabilizers according to the present invention are described.

Stabilizer A

Preparation of2-hydroxy-2-oxo-3-amino-3-phosphonyl-5-oxo-1,2-azaphosphacyclopentane,n=1. ##STR5##

102 gm of malonic acid diamide (1.0 mol) and 164 gm of H₃ PO₃ (2.0 mols)were melted at 70° C. with exclusion of moisture and 175 ml of PCl₃ (2.0mols) were slowly added while stirring. A viscous yellow mass was formedwhich after standing for four hours was hydrolyzed with 1 liter of H₂ O.After addition of activated charcoal and filtering, the filtrate wasconcentrated to 400 ml and precipitation was effected with 4 liters ofethanol to give a white crystalline substance. The substance was driedat 50° C. in a vacuum drying cabinet. The crude yield was 142 gm, orabout 53% of theory.

The compound was first isolated as dihydrate, which by the titrimetricmethod had a molecular weight of 266 (calculated 266.1).

After more rigorous drying the water-free compound was obtained with amolecular weight of 228 (calculated 230).

In the infra-red spectrum the substance showed a γ_(CO) bend at 1670cm⁻¹ and a δ_(NH) band at 1615 cm⁻¹. It melted at 180° C. withdecomposition.

Stabilizer B

Preparation of2-hydroxy-2-oxo-3-amino-3-phosphonyl-6-oxo-1,2-azaphosphacyclohexane,n=2. ##STR6##

232 gm of succinic acid diamide (2.0 mols) and 328 gm of H₃ PO₃ (4.0mols) were melted at 70° C. with exclusion of moisture and 350 ml ofPCl₃ (4.0 mols) were slowly added thereto under stirring. After 4 hoursthe viscous yellow mass formed was hydrolyzed with 2 liters of H₂ O. Thesolution was treated with activated charcoal and filtered. The filtratewas concentrated to 500 ml and precipitation was effected with 3 litersof ethanol and 3 liters of acetone to give a white crystallinesubstance. Crude yield 180 gm, about 35% of theory.

The compound was first obtained as the monohydrate, and thetitrimetrically determined molecular weight was 260 (calculated 262.1).

After drying at 80° C. in vacuo the anhydrous cyclic compound wasobtained with a molecular weight of 244 (calculated 244). The infra-redspectrum showed a very broad γ_(CO) band at 1640 cm⁻¹, by which theδ_(NH) band was masked. It melted at 320° C. with decomposition.

Stabilizer C

Preparation of2-hydroxy-2-oxo-3-amino-3-phosphonyl-7-oxo-1,2-azaphosphacycloheptane,n=3. ##STR7##

55 gm of glutaric acid diamide (0.42 mol) and 140 gm of H₃ PO₃ (1.7mols) were melted at 70° C. and then 149 ml of PCl₃ (1.7 mols) wereslowly added thereto. After a further 4 hours at 80° C., the mixture washydrolyzed with 400 ml of H₂ O and the hot solution was treated withactivated charcoal and filtered. The white cyclic diphosphonic acid wasprecipitated from the filtrate with ethanol and acetone. Crude yield 40gm, about 35% of theory.

After drying a short time at 50° C. the substance was obtained asmonohydrate. The molecular weight was determined titrimetrically as 276(calculated 276). After drying at 80° C. in vacuo the anhydroussubstance was obtained with a molecular weight of 260 (calculated 258).The product melted at 285° C. with decomposition. In the infra-redspectrum, the γ_(CO) band was at 1660 cm⁻¹ and the δ_(NH) band was at1615 cm⁻¹.

The following examples serve to demonstrate the superior stabilizingactivity of the cyclic aminophosphonic acids to be used as stabilizersaccording to the present invention. In these examples, the productsstabilized according to the present invention were not only comparedwith untreated anhydrous dibasic calcium, but also compared withproducts which were obtained by treatment with other structurallydifferent phosphonic acids.

In order to be able to compare truly the individual experiments, it isnot only necessary during the measurements to keep constant, forexample, the parameters of temperature, pH, amount of anhydrous dibasiccalcium and amount of solution, but also the same anhydrous dibasiccalcium phosphate must always be used.

For the investigations a relatively large amount of crystallineanhydrous dibasic calcium phosphate was prepared from calcium hydroxideand phosphoric acid which had the following particle size distribution:

    ______________________________________                                        Particle                                                                      Size in μ      Percentage                                                  ______________________________________                                        >5                24                                                           5 to 10          43                                                          10 to 20          19                                                          <20               14                                                          ______________________________________                                    

The preparation of the anhydrous dibasic calcium phosphate was carriedout in the following way:

4.9 kg of 40% phosphoric acid were placed in a closed, heatable stirredapparatus and heated to 100° C. To this over the course of about 4 hoursa suspension of 1.32 kg of calcium hydroxide and 4.12 kg of water wereadded dropwise. The phosphate formed was filtered off by suction,suspended again in water, and the suspension was made neutral withhydrochloric acid, filtered again and dried. The yield of anhydrousdibasic calcium phosphate was 2.2 kg.

EXAMPLE 1

32.5 gm of an anhydrous dibasic calcium phosphate having a particle sizedistribution up to 30μ, prepared as previously described, were treatedin 100 ml of a phosphate buffer solution at a pH of 7.5 with 97.5 mg of3-amino-2-hydroxy-2,7-dioxo-3-phosphono-1,2-azaphosphacycloheptane (0.3%by weight based on the phosphate). The suspension was held for 24 hoursat room temperature, then filtered and dried.

30 gm of the product thus stabilized were then suspended at 40° C. in100 ml of a 10⁻¹ molar sodium fluoride solution, prepared by dissolvingsodium fluoride in phosphate buffer solution at pH 7.

The decrease of the fluorine ion content of the solution to 10% of thestarting value was followed and the time recorded (Orion ResearchDigital-pH/mV, Model 701 with recorder).

The experiment was so arranged that over the experimental period noconcentration of the solution by evaporation of the solvent could takeplace.

Similarly samples were measured which were treated for stabilizationwith the following phosphonic acids:

0.3% amino-tris-(methylenephosphonic acid)

0.3% 1-hydroxyethane-1,1-diphosphonic acid

0.3% ethylenediamine-tetra-(methylenephosphonic acid)

0.3% methylphosphonosuccinic acid (2-phospho-propane-1,2-dicarboxylicacid).

Moreover, a sample was measured which had been similarly prepared, butwithout addition of a phosphonic acid stabilizer.

The measured results obtained are shown in the following Table 1.

                  TABLE 1                                                         ______________________________________                                                              Fluorine Ion                                            Anhydrous CaHPO.sub.4 Treated                                                                       Reduction to                                            With 0.3% of the following                                                                          10% in Hours                                            ______________________________________                                        3-Amino-2-hydroxy-2,7-dioxo-                                                  3-phosphonyl-1,2-azaphospha-                                                                        2.0                                                     cycloheptone                                                                  Amino-tris-(methylenephosphonic                                               acid)                 0.25                                                    1-Hydroxyethane-1,1-diphosphonic                                              acid)                 0.8                                                     Ethylenediamine-tetra-(methylene-                                             phosphonic acid)      0.66                                                    Methylphosphonosuccinic acid                                                                        0.2                                                     Untreated             0.2                                                     ______________________________________                                    

The results show clearly the superior stabilizing action of the cyclicaminophosphonic acids to be used according to the present invention.

EXAMPLE 2

Anhydrous dibasic calcium phosphate samples which had been treated with1.25% of the phosphonic acids mentioned in Example 1 were prepared andmeasured, according to the particulars given in Example 1. The followingTable 2 contains the results.

                  TABLE 2                                                         ______________________________________                                                              Fluorine Ion                                            Anhydrous CaHPO.sub.4 Treated                                                                       Reduction to                                            With 1.25% of the following                                                                         10% in Hours                                            ______________________________________                                        3-Amino-2-hydroxy-2,7-dioxo-3-                                                phosphonyl-1,2-azaphosphacyclo-                                                                     312                                                     heptane                                                                       Amino-tris-(methylenephosphonic                                               acid)                 0.4                                                     1-Hydroxyethane-1,1-diphosphonic                                              acid                  1.8                                                     Ethylenediamine-tetra-(methylene-                                             phosphonic acid)      0.6                                                     Methylphosphonosuccinic acid                                                                        0.2                                                     Untreated             0.2                                                     ______________________________________                                    

The example shows still more clearly than Example 1 the superiorstabilizing action of the cyclic aminophosphonic acids to be usedaccording to the present invention.

EXAMPLE 3

An anhydrous dibasic phosphate sample was prepared and measuredaccording to Example 1, which had been treated with 1.25% of3-amino-2-hydroxy-2,6-dioxo-3-phosphonyl-1,2-azaphosphacyclohexane. Thereduction of fluorine ion to 10% of the starting value did not occuruntil after 321 hours.

EXAMPLE 4

Two similar toothpastes were prepared. One contained untreated anhydrousdibasic calcium phosphate as a polishing material, and the other ananhydrous dibasic calcium phosphate treated with 0.75% of3-amino-2-hydroxy-2,7-dioxo-3-phosphonyl-1,2-azaphosphacycloheptane.Both phases also contained 0.1% of fluoride as sodium fluoride.

After 16 days storage at 50° C., soluble fluoride could no longer bedetected in the paste with untreated polishing material, while in thepaste with stabilized polishing material 68% of the fluoride was stillpresent after 50 days.

In the following examples formulations for tooth cleaning preparationsare given which contain anhydrous dibasic calcium phosphate stabilizedaccording to the present invention as a polishing material.

EXAMPLE 5

Composition of a toothpaste according to the present invention.

    ______________________________________                                                              Parts by                                                                      Weight                                                  ______________________________________                                        Glycerine               30.0                                                  Water                   18.0                                                  Sodium carboxymethylcellulose                                                                         1.0                                                   Anhydrous dibasic calcium phosphate                                           stabilized with 1% of 2-hydroxy-                                              2,7-dioxo-3-amino-3-phosphonyl-                                               1,2-azaphosphacycloheptane                                                                            36.0                                                  Insoluble sodium metaphosphate                                                                        10.0                                                  Sodium lauryl sulfate   1.0                                                   Pyrogenic silicic acid  1.5                                                   Sodium monofluorophosphate                                                                            0.5                                                   Essential oils          1.5                                                   Saccharin sweetener     0.5                                                   ______________________________________                                    

Instead of Stabilizer C used in the above formulation for stabilizingthe anhydrous dibasic calcium phosphate, Stabilizers A and B can be usedin the same amounts with the same good results.

EXAMPLE 6

Composition of a toothpaste according to the present invention.

    ______________________________________                                                              Parts by                                                                      Weight                                                  ______________________________________                                        Anhydrous dibasic calcium phosphate                                           stabilized with 1% of 2-hydroxy-                                              2,5-dioxo-3-amino-3-phosphonyl-                                               1,2-azaphosphacyclopentane                                                                            40.0                                                  Water                   29.3                                                  Glycerine               18.0                                                  Sorbitol                7.0                                                   Sodium carboxymethylcellulose                                                                         1.0                                                   Pyrogenic silicic acid  1.5                                                   Sodium lauryl sulfate   1.0                                                   Essential oils          1.5                                                   Saccharin sweetener     0.5                                                   Sodium fluoride         0.2                                                   ______________________________________                                    

Instead of the Stabilizer A used in the above formulation for thestabilization of the anhydrous dibasic calcium phosphate, Stabilizers Band C may be used in the same amounts with the same good results.

EXAMPLE 7

Composition of a tooth powder according to the present invention.

    ______________________________________                                                              Parts by                                                                      Weight                                                  ______________________________________                                        Anhydrous dibasic calcium phosphate,                                          stabilized with 1% of 2-hydroxy-                                              2,7-dioxo-3-amino-3-phosphonyl-                                               1,2-azaphosphacycloheptane                                                                            50.0                                                  Precipitated chalk      30.0                                                  Finely divided silicic acid                                                                           10.0                                                  Milk sugar              4.0                                                   Precipitated magnesium carbonate                                                                      3.5                                                   Titanium dioxide        1.0 -Tannin 1.0                                       Sodium monofluorophosphate                                                                            0.5                                                   ______________________________________                                    

Instead of the Stabilizer C used in the above formulation for thestabilization of the anhydrous dibasic calcium phosphate, Stabilizers Aand B may be used in the same amount with the same good results.

The preceding specific embodiments are illustrative of the practice ofthe invention. It is to be understood, however, that other expedientsknown to those skilled in the art or disclosed herein, may be employedwithout departing from the spirit of the invention or the scope of theappended claims.

We claim:
 1. A process for stabilizing anhydrous dibasic calciumphosphate against reaction with fluorine ions consisting essentially ofsuspending anhydrous dibasic calcium phosphate in an aqueous medium at apH of from 5 to 10, containing from 0.01% to 5% by weight based on thecontent of dibasic calcium phosphate of a cyclic amino-phosphoniccompound selected from the group consisting of (A) compounds of theformula ##STR8## wherein n is an integer from 1 to 3, and (B)water-soluble salts thereof, and separating said stabilized anhydrousdibasic calcium phosphate.
 2. The process of claim 1 wherein said pH isbetween 6 and
 8. 3. The process of claim 1 wherein the amount of saidcyclic aminophosphonic acid compound is from 0.1% to 2% by weight basedon the content of dibasic calcium phosphate.
 4. The process of claim 1wherein said water-soluble salts are selected from the group consistingof the alkali metal salts and ammonium salts.
 5. An anhydrous dibasiccalcium phosphate stabilized against reaction with fluorine ions by acyclic amino-phosphonic acid produced by the process of suspendinganhydrous dibasic calcium phosphate in an aqueous medium at a pH of from5 to 10, containing from 0.01% to 5% by weight based on the content ofdibasic calcium phosphate of a cyclic amino-phosphonic compound selectedfrom the group consisting of (A) compounds of the formula ##STR9##wherein n is an integer from 1 to 3, and (B) water-soluble saltsthereof, and separating said stabilized anhydrous dibasic calciumphosphate.